Airscrew



June 8, 1937. A. H. k. FEDDEN ET AL 3 AIRSCREW 2 Sheets-Sheet 1 FiledJuly 31, 1935 e: 010%) u h e- June 8, 1937.

AIRSCREW Filed July. 31, 1955 A. H. R. FEDDEN ET AL WWW- 2 Sheets-Sheet2 InvnEnS' I filfrad ZZ fil s'ddew Frank M 02mm Patented June a, 1931UNITED srATEs AIRSCBEW Alfred Hubert Roy Owner, Bristol,

Fedden and Frank Morgan England Bristol Aeroplane Company, Limited,Bristol,

, assignors to The England, a British company Application July 31,1935,Serial No. 34,084 In Great Britain September 21, 1934 4 Claims.(Cl. 170-173) This invention is for improvements in or relating toairscrews for aircraft and has for its object to provide an improvedconstruction-for the root of a metal airscrew blade whereby such bladesare rendered more suitable for usev in adjustable-pitch andvariable-pitch airscrews. The term adjustable-pitch airscrew", as usedherein, means an airscrew, the blade-pitch of which may be set to adesired value when the aircraft is on the ground and the airscrew isstationary, but cannot be adjusted during flight. The expressionvariable-pitch airscrew, as used herein, means an airscrew of which thebladepitch can be varied while the airscrew is rotating.

In an adjustable-pitch orvariable-pitch airscrew in which the airscrewblades are of a comparatively soft metal such as a magnesium alloy oraluminium alloy, it is necessary to provide reinforcement at the root ofthe blade for the stresses to which the blade is subjected. According tothe present invention, an airscrew blade of comparatively soft metal hasa sleeve of harder metal screw-threaded and shrunk on to the blade-rootin such manner that the sleeve grips the root over thewhole of theco-operating screw-threads.

According to another feature of the invention,

a detachable flange is provided (e. g. on an outer sleeve) surroundingthe sleeve to constitute an abutment for a thrust hearing within theairscrew hub. An airscrew-blade as set out above may comprise a coatingon one or both of the inter-engaging surfaces of a thin layer of softmaterial to prevent fretting between the surfaces.

Specific embodiments of the present invention will now. be described. byway of example, with reference to the accompanying drawings, of which:-

Figure 1 is an elevation, partly in section, of a three-bladedadjustable-pitch airscrew hub showing the root portions of the blades:

Figure la is a section of a portion of the airscrew blade and itssurrounding sleeve, drawn to a greatly enlarged scale, the section beingtaken on the line la-la of Figure 1; I

Figure 2 is a sectional elevation on the line '2.2 of Figure 1 withcertain parts omitted;

Figure 3 is a sectional elevation of part of the blade-root and part ofthe hub of a variablepitch airscrew according to the invention;

Figure 4 shows an alternative form of Figure 3, and

Figure 5 showsan alternative form of bladeretaining nut.

As shown first in Figures 1 and 2, the hub ll) of the airscrew is ofone-piece construction, as opposed to the known form of split hub oom- Iprising two halves clamped together to embrace the shanks of the blades.Each of the airscrew blades II is of magnesium alloy having thefollowing compositiom- Percent Aluminium-not more than l0 Zinc-not morethan -r- 1.5 Manganese-not more than 1 Impurities-not more than 1.5Magnesiumthe remainder.

Alternatively, the blade may be composed of an alloy in which aluminiumpredominates.

The inner end of each blade II is screw-. threaded as shown at l2 and isengaged by a sleeve l3 of comparatively hard steel alloy. The 1 steelsleeve is screw-threaded and shrunk on to the blade. The diameter of thescrew-threads in the sleeve I3 is somewhat less than the di-' ameter ofthe screw-threads on the blade I l prior to assembly. The sleeve isheated, and thereby caused to expand, and is screwed on to the root ofthe blade while still hot. When the sleeve cools, the blade is tightlygripped by a shrink fit over the whole of the co-operating screwthreads.The expression shrink fit as herein used; means the grip obtained bydifferential heating of the two parts. y

Where the blade is composed of the magnesium alloy above-specified, asuitable diameter for the inside of the sleeve, when cold, is found tobe .001", per inch of diameter, less {than the external diameter of theblade threads.

The threads are formed to constant dimensions on each part and may be sodesigned that 40 the radius of curvature at the bottom of the thread onthe blade is greater than the radius at the bottom of the thread on thesleeve, which is of harder metal.

It will be seen that the end of the sleeve is tapered; this tapered endalso tightly grips the blade-so that the blade and the tapered portiontend to bend as one part and localization of stress is avoided.

The sleeve i3 is formed with an integral flange H which rests on ashoulder I! in the hub socket and the blade is retained in the hub by anut l8 engaging the flange ll. 7

It will be seen that the angular position of the blade I l in the hubmay be adJusted as desired 66 substance smeared on to 2 .whentheaircraftisonthegroundby slackening the nut It, rotating theblade to thedesired poand subsequently tightening up the nut. Thesteel sleeveadequately reinforces the com- ,paratively soft metalof the blade andprovides a suiilciently hardsu'rface for the intense centrifugal "andlateral'stresses which occur, as is well known. between the root hubsocket. 10 In the alternative-construction shown in l"lg. 4 "ure 5,-asleeve if of hard metal is screw-threaded and shrunk on to the root ofthe blade H, which is of'comparatively soft-metal, and the flange ll ofthe blade-sleeve l8 rests against the l8 outer end of the hub-socketand-is retained by a nut ll screw-threaded on to the outer surface ofthe hub-socket. I

It is known that, where two stressed metal parts are fitted together,inter-molecular action 20 between the parts gives rise to roughening ofthe enga ing surfaces although movement between them. known asfretting'.'.

ting between the root of the blade and-the sur- 3 rounding sleeve, theen aging surfaces between these two parts, according to this invention,are coated with a thin layer of a protective substance, before assembly.The protective substance may be oil or some resinous or gummy theengaging surfaces. Alternatively, the surfaces may be electro-platedwith a very thin layer, 'of the order of afew tenthousandths of an inchthick, oi. soft metal such as copper or tin.. Such a layer of materialbetween the sleeve II and the root of the blade II is shown as 40 inFigure 1a and prevents fretting between these two parts. In the modifiedconstruction shown in Figure 3, which shows only a part of the rootof asoft inetalblade n, the steel sleeve is for the blade is expanded byheat and screw-threaded on to the blade so that when the parts are coldit engages the blade by a shrink fit, and an intermediate layer 40 ofsofter material as shown by Fig. la is provided as in the examplealready described. A- flanged ringit is screw-threaded on to the outersurface of the sleeve I! by means of a nut portion 21 to form anabutment surface for one of the races 41 Ma roller bearing havingrollers It. The other race 48 of the bearing abuts against a flangedring 2. screw-threaded into the hub-socket II. I

The flanged ring ll has an inward cylindrical extension II which forms abearing with a cooperating face 22 on the inside of the hub. -At a pointnear its outer end, the steel'sleeve II has a cylindrical face 23 whichforms another bearing with an outward cylindrical extension 14 n of theflanged ring II. Rings 2| and II are time This phenomenon is vided toprevent the escape of oil from the mech- I anism: in addition the sleeveit carries a sealing.

of the blade and the there is no relative In order to preventfretspecttotheblade.

ALI'RID alternative 7 in the flange is itself constitutes ones-acethrustrollersil-andheatendsdinwardlyfltoeonstitutetheinnerraeeof'arollerbearing Is. The otherraee'wll'andthe are provided asin Figure 3: also.

is of soft new and the We claim; 1. A reinforced all-screw bladecylindrical root-portion of metal. an screw-thread formed root-portion.a sleeve of round and reinforce said root-portion, a thread formedon theinternal surface of said sleeve and engaging, by a shrink-fit over itsentire length, with the screw-thread on the said root-portion.

on hardermetaltosur' serew-,

2. Anairscrewbladeof rnagnelhlmalloyhav thread on the motortion of 4.Areinforcedairscrewbladeofcompara flw,

1y soft metal having a asleeve of harder metal w l. by a shrink-ht Ialong the whole of its length. with mill root-portion, and bt'erengaslnsscrew-threads between the sleeve and the. root-potion of the blade.whereinthem'etalsleeveisirmnovablewithre- 'mauxuoaosirowm summits-mom;

